Conventional medium conveyance mechanisms (devices) handle tickets of different sizes, and are sized such that the width of opposing belts corresponds to the width of large tickets. Accordingly, in the case of conveying a large ticket by the opposing belts, the large ticket is conveyed so that a side thereof follows the surface of a guiding wall provided at a side of the opposing belts. When a small ticket is conveyed, on the other hand, because a space exists between the small ticket and the guiding wall surface, the small ticket is pressed against the guiding wall surface by an aligning device so that an edge of the small ticket follows the guiding wall surface. (See, for example, paragraph [0011] and FIG. 1 of Japanese Patent Application Laid-Open (JP-A) No. 2002-249263.)